It has been conventional practice to prepare various textile reinforcing fibers to be used in contact with rubber formulations by pretreating them with a mixture of a rubber latex and a phenol-formaldehyde resin in which the phenol has almost always been resorcinol. This is the so called "RFL" (resorcinol-formaldehyde-latex) method. Another method commonly used is to generate the resin in situ (in the vulcanized rubber/textile matrix) by incorporating in the rubber a formaldehyde (or methylene) donor compound, for example, hexamethylenetetramine or hexamethoxymethylmelamine and a formaldehyde (or methylene) acceptor compound, for example, a dihydroxybenzene compound such as resorcinol. The in situ method has been found to be particularly effective where the reinforcing material is brass coated steel wire since pretreatment of the wire with the RFL system has been observed to be largely ineffective.
The in situ formed resins have been observed to enhance adhesion between the rubber and the reinforcing material. This enhancement of adhesion is by a mechanism that is not completely understood. The in situ method which entails compounding a vulcanizable rubber stock with the phenol-formaldehyde components is known in the art. The most commonly employed methylene acceptor is resorcinol, while the more commonly employed methylene donors are the N-substituted oxymethylmelamines. The resin is formed in situ during vulcanization of the rubber creating a bond between the fiber and the rubber irrespective of whether the fiber has been pretreated or not.
The use of resorcinol has some inherent properties which can be disadvantageous. One of the most frequently encountered is occasioned by the fact that resorcinol is not readily dispersed in rubber. In addition, resorcinol in its raw form is excessively volatile. These disadvantages are further enhanced because, on occasion, there is a worldwide shortage of resorcinol.
The rubber industry utilizes resorcinol in conjunction with methylene donors to form resins which are used in rubber compounds to enhance filamentary reinforcement and to improve the physical properties, including aged physical properties, of tires and other compounded rubber formulations. These resins, whether preformed or formed in situ, can increase the dynamic rubber stiffness, tire treadwear, interfacial adhesion, and fabric adhesion to the rubber compound; while desirably maintaining the values of and hopefully reducing the rolling resistance and heat build-up of a tire.
In spite of numerous attempts to replace resorcinol, no direct resorcinol replacement or single resin system substitute is known to be used in the manufacture of commercial rubber products to date.
An aspect of the instant invention resides in the use of specific phenolic esters as the methylene acceptor which has unexpectedly been observed not to substantially interfere with the curing of the rubber and has further unexpectedly been observed to enhance the physical properties of the final rubber vulcanizate. In general, the instant invention provides a new and improved vulcanizable rubber stock which avoids certain disadvantages of employing resorcinol as the methylene acceptor, as well as providing equal or better rubber to reinforcement bonding results.
U.S. Pat. No. 3,517,722 discloses a pneumatic tire comprising polymer, a methylene donor, a methylene acceptor, wherein the methylene donor and acceptor may be in free or complex form, with about 0.5 to about 1.5 parts by weight of sulfur wherein the methylene donor is selected from the group consisting of hexamethylenetetramine and complexes thereof, N-(substituted oxymethyl) derivatives of urea, N-(substituted oxymethyl) melamine derivatives and azomethyne compounds.
U.S. Pat. No. 4,338,263 is concerned with vulcanizable rubber compositions providing enhanced bonding of textile or metal reinforcing fibers thereto which comprises a rubber, a filler material, and an N-(substituted oxymethyl) melamine, and a 4,4'-isopropylidenediphenol reaction product or solution.
U.S. Pat. No. 3,586,735 discloses the use of Bisphenol A in a resin which is added to rubber.
U.S. Pat. No. 4,339,359 discloses a method for adhering rubber to reinforcing materials which comprises embedding a textile fiber or metal reinforcing material in a vulcanizable rubber composition comprising rubber, a sulfur vulcanizing agent, high surface area silica and an N-(substituted oxymethyl)melamine compound, thus apparently eliminating resorcinol or other methylene acceptor compounds.
U.S. Pat. No. 3,992,334 discloses a vulcanizable rubber composition which comprises rubber, a filler, an N-(substituted oxymethyl)melamine and a 1,3-dimethylolurea.
U.S. Pat. No. 4,038,220 discloses a vulcanizable rubber composition which comprises rubber, a filler, an N-(substituted oxymethyl)melamine and at least one of alpha or beta naphthol.
U.S. Pat. Nos. 2,571,703 and 2,588,978 relate to processes for the production of resorcinol monobenzoate. Resorcinol monobenzoate is presently available on a commercial scale.
None of the prior art suggests or discloses that the resorcinol component of the resin used in RFL adhesive systems or as in situ resins in rubber, may be replaced by phenolic esters such as resorcinol monobenzoates or resorcinol rosinates.